In Paris (France), the blue line refers to the subway line 2, which counts 25 stops from PORTE DAUPHINE to NATION. It connects the west to the east of the city, passing by the northern districts. It takes 35 minutes from an end to another. It is approximately 12.5 km long.
A blues line refers to the lyrics of a song. The song Hard Time Killing Floor Blues by Skip James, recorded in 1931, counts 16 lines :
“Hard time here and everywhere you go
Times is harder than ever been before
And the people are driftin' from door to door
Can't find no heaven, I don't care where they go…”